1. Field of the Invention
The present invention relates to a rewritable thermal recording medium.
2. Description of the Related Art
In recent years, with the advance of office automation, the amount of various information has significantly increased, and the chances of information output have also been increased with increase in the information amount. In general, the information outputs are classified into a hard copy output from a printer to paper sheets and a display output. Unfortunately, in the hard copy output, a large quantity of paper is consumed as a recording medium with increase in the information output amount. Therefore, the hard copy output is expected to be a problem in the future in respect of protection of natural resources. On the other hand, the display output requires a large scale circuit board in a display unit. This brings about problems of portability and cost. For these reasons, a rewritable recording medium, which is free from the above-noted problems inherent in the conventional technique, is anticipated as a third recording medium.
Some recording materials for such a rewritable recording medium, which contains a color former such as a leuco dye and various acids acting as a developer, are certainly known to the art. In the conventional recording material of this type, a color development and decoloring are brought about by the interaction between the color former and the developer. For example, Japanese Patent Disclosure No. 4-50290 discloses recording materials which contain a leuco dye, an acid as a developer, and a long-chain amine as a decoloring agent, and in which heat energy is supplied to the recording material so as to repeatedly perform the chemical color development and decoloring. Additional recording materials, which contain a leuco dye and a long-chain phosphonic acid as a developer and in which the heat energy is controlled so as to change the crystal structure and, thus, to achieve reversible changes between the color developed state and the decolored state, are disclosed in, for example,"Pre-prints for 42nd Polymer Forum (1993, page 273)". Further,"Japan Hardcopy '93, pp 413-416" teaches an additional type of recording material, which contains a leuco dye and a long-chain 4-hydroxyanilide compound that is highly crystallizable and in which reversible changes between the color developed state and the decolored state are achieved, by supplying heat energy, on the basis of reversible changes between the crystalline state and amorphous state.
However, these recording materials require a very large activation energy for repeatedly performing the color development-decoloring functions, making it difficult to improve the recording-erasing speed in general. Where, for example, the color development and decoloring of the composition containing a color former and a developer are reversibly repeated on the basis of the transition between the crystalline state and the amorphous state, it is difficult to improve the recording-erasing speed, because the transition from the amorphous state, which is in a metastable nonequilibrium state, to the crystalline state, which is in a stable equilibrium state, takes a long time in general. On the other hand, where the composition is prepared simply in view of improvement in the speed of the transition from the amorphous state to the crystalline state, it is difficult to allow the composition to form an amorphous state of a long life. In other words, if it is intended to improve the recording-erasing speed in the conventional recording material containing a color former and a developer, the color developed state or the decolored state, whichever corresponds to the metastable nonequilibrium state, is rendered unstable. As a result, a change from the nonequilibrium state to the equilibrium state proceeds gradually even if a heat energy is not supplied form the outside to the recording medium, giving rise to reduction in the thermal stability of recording.
As described above, vigorous studies are being made on a rewritable thermal recording medium using a recording material containing a color former and a developer. In the conventional thermal recording medium, however, an improvement in the recording-erasing speed is contradictory to an improvement in the thermal stability of recording. Since it is difficult to satisfy these requirements simultaneously, a satisfactory rewritable thermal recording medium has not yet been put to a practical use.